Generally, metal powder injection molding for producing sintered products is composed of the sequential steps of (i) injection molding in which a binder is added to a metal powder material to provide pressure-moldability thereto and then the mixture is formed into a molded form; (ii) degreasing for removing the binder from the molded form; and (iii) sintering of the degreased, molded form. Such metal powder injection molding has advantages over other methods such as the metal powder press molding process in that products of intricate shapes can be produced through a single process and that less post-treatments are required. These characteristics are not only suited for the production of small-sized metal parts and but also economically advantageous.
Powder injection molding however presents a disadvantage. That is, since this technique uses large amounts of binder for providing hydrostatic pressure moldability, collapsing, blistering and cracking are more likely to occur in the resultant, molded products particularly in the steps of degreasing and sintering compared to other ordinary powder metallurgical processes. In order to prevent such collapsing, blistering and cracking, removal of the binder has to be proceeded slowly in the degreasing step, so that it usually takes two or three days to perform the degreasing step, although the time required for it principally depends on the shape and thickness of molded products to be produced. Collapsing, blistering, cracking and the prolonged degreasing step are the outstanding problems for powder injection molding.
There have been proposed several techniques to eliminate various defects as mentioned above during removal of a binder from injection-molded products. One example of such techniques is disclosed in Japanese Patent Publication No. 61-48563 (1986) in which degreasing is carried out with a turbulent flow of inert gas blown to injection-molded products and part of binder components dissolved in a porous form is absorbed. Japanese Patent Publication No. 62-33282 (1987) discloses a degreasing method for maintaining the pressure in the atmosphere for injection-molded products to be no less than the vapor pressure of the thermoplastic binder.
However, it is practically difficult to keep the interior of a degreasing furnace in a uniform turbulent condition. Even if temperature can be kept uniform, the binder removing speed on the side of an injection-molded product exposed to blown air differs from that on the side opposite to the exposed side, with the result that the degree of degreasing undesirably varies within a single piece of injection molded product. When a setter is used for effective arrangement of injection molded products in a degreasing furnace, non-uniform degreasing is often seen in areas contacting the setter or areas from which a turbulent flow of air is shut out. Such non-uniformity is remarkable particularly when a plurality of injection molded products are processed in a binder removal furnace, resulting in variations in the degree of degreasing between the injection molded products. For the above reason, it is practically infeasible to produce sound degreased products free from defects such as deformation due to the inherent weight of the products themselves, blister and cracks.
It is also contemplated in the above publication that a thermoplastic binder composed of binder components of different melting points is used and these components are allowed to sequentially, gradually flow out from the injection molded products in the form of liquid to be absorbed by a porous absorber so that the removal of the binder can be promoted while preventing occurrence of blistering and cracking. However, this method is rather impractical in consideration of the problems that (i) molded products tend to fail in withstanding their own weight during the degreasing step, resulting in collapse and that (ii) the progress of degreasing varies within a molded product similarly to the above case when the binder absorber is disposed in partial contact with the molded product, and as there are some requirements for arranging a plurality of molded products within a degreasing furnace, it is difficult to carry out stable degreasing to obtain a number of acceptable molded products free from deformation due to their own weight, blister and cracks.
There have been made proposals for overcoming the defects of the heat degreasing methods such as described above and the prolonged degreasing process. For instance, U.K. Patent No. 1516079 discloses that a molded product is formed with use of a binder composed of (i) a water-soluble polymer such as polyethylene glycol, polypropylene glycol and polyvinyl alcohol and (ii) a water-insoluble polymer such as polystyrene and polyethylene, and that the water-soluble polymer is preferentially extracted using water as a solvent and then the remaining polymer is removed for example by heating. One example disclosed in the above U.K. patent is such that a molded product produced with a binder comprising three components (i) polyethylene glycol (water-soluble polymer), (ii) polystyrene (to be extracted using methylene chloride as a solvent) and (iii) polyethylene (a strength component to be removed by heating) is first heated to a temperature equal to the melting point of polyethylene glycol or more. The water-soluble polymer is extracted using water as a solvent and then, the polystyrene polymer is extracted with methylene chloride. Thereafter, the molded product is further heated thereby finally removing the strength component, i.e., polyethylene from the molded product.
In this example, polyethylene that imparts strength to a molded product and can be extracted by a solvent is used in addition to polyethylene glycol because polyethylene glycol is poor in strength and use of it in large amounts weakens a molded product. This involves two solvent-extraction steps (one step is carried out with water and the other is with methylene chloride) and as a result, degreasing time cannot be satisfactorily reduced. Furthermore, use of an organic solvent such as methylene chloride is unavoidable which leaves financial problems as well as environmental problems to workers.
According to the above method, since water solvent extraction is performed by use of vapor heated to about 100.degree. C. after the molded product is heated to a temperature equal to the melting point of polyethylene glycol or more, the molded product swells as the binder captures water. In addition, since polyethylene glycol has the properties of a water-soluble binding agent, the aqueous solution containing a high percentage of polyethylene glycol dissolved therein exhibits high viscosity and this disallows quick, uniform removal of polyethylene glycol from the entire surface of the molded product so that "sagging" similar to a sweating phenomenon is likely to occur. These defects lead to occurrence of cracking and deformation in the heat degreasing step which follows the water solvent extraction.
Techniques substantially identical to U.K. Patent No. 1516079 are disclosed in Japanese Patent Publication Laid-Open Nos. 2-101101 (1990), 2-182803 (1990), 2-182804 (1990) and 2-305903 (1990). As the water-soluble polymer typically used in powder molding which corresponds to polyethylene glycol, polypropylene glycol and polyvinyl alcohol of U.K. Patent No. 1516079, these publications use polyethylene glycol such as described in Powder Molding Hand Book (issued by The Nikkan Kogyo Shinbun Ltd. on Feb. 1987), macromolecular polyethylene oxides, methyl cellulose, carboxyl methyl cellulose (CMC), polyacrylamide and polyvinyl ether. They use a water-insoluble polymer such as polyethylene and polystyrene, like U.K. Patent No. 1516079.
More specifically, according to the above publications, a polyethylene oxide that is a high-molecular water-soluble polymer is used as a binder component in order to compensate for the weakness of the above-mentioned water soluble polymer, i.e., polyethylene glycol. After part or all of the polyethylene oxide is removed by water solvent extraction, most of the remaining polyethylene binder is removed by heating. Thermoplastic polyethylene oxides are, however, known to be poor in thermal stability. For example, when a binder containing a polyethylene oxide is mixed with a metal powder (SUS430c) by heating at 150.degree. C., decomposition of the polyethylene oxide continuously takes place because of shear heat generated during mixing, so that stable mixing torque cannot be ensured. According to the result of a thermal analysis conducted on a binder which has been mixed with a powder material and kneaded until the mixture comparatively stabilizes, the temperature of the mixture drops from the average melting point (i.e., about 70.degree. C.) of polyethylene oxides to about 54.degree. C. that is equal to the melting point of polyethylene glycol. It can be assumed from this fact that most of the polyethylene oxide is changed to lower-molecular polyethylene glycol.
As described above, the methods of the above publications have difficulty in obtaining stable flowability in the mixed material and return row material and suffer from deterioration in the strength of the binder which is resulted from the decomposition of a polyethylene oxide. There are other problems which limit the shape of molded products to be produced, these problems including: variations in the weight of molded products caused by the unstable flowability of material; variations in dimensional accuracy after sintering; damage to products when taken out of molds owing to the decreased strength of the binder; and cracking of molded products during the step for cooling the inside of molds. Furthermore, the corners of molded parts are susceptible to collapse during the water solvent extraction because of the resistance of flowing water being gently agitated. During the water solvent extraction, molded products slightly swell throughout the entire area and swelling is considerable especially at their parting line sections. This is a fatal defect for some molded parts. Another problem is that the effects of raised temperature for increasing the rate of extraction cannot be expected, because the temperature of water in the water solvent extraction should be kept at about 50.degree. C. or less in order to prevent "sagging" similar to a sweating phenomenon.
According to the above publications, the mechanism of the extraction is such that the water-soluble polymer at the surface of a molded product is first dissolved and diffused in water and at the same time, water penetrates into the molded product through the voids in which the dissolved water-soluble polymer has existed. The water-soluble polymer present in the vicinity of the void passages in the molded product is dissolved into the penetrating water and the water-soluble polymer thus dissolved then scatters in water that exists outside the molded product. The extraction rate for the water-soluble polymer in this process is discussed in connection with the relationship between the elementary process (I) where the rate of the movement/diffusion of the water-soluble polymer within the void passages of the molded product is controlled and the elementary process (II) where the difference in the concentration of the water-soluble polymer dissolved in the solvent between at the surface of the molded product and at the area remote from the above surface and the rate of the diffusion of the water-soluble polymer into the water solvent are controlled. It is known that polyethylene oxides, which are regarded as suitable water-soluble polymers in the publications, have good properties as high-viscosity water-soluble binding agents, and for instance, they are completely gelled in a 1 wt % aqueous solution at room temperature. For the reasons of (i) the properties of polyethylene oxides, (ii) the fact that the water-soluble polymer is dissolved at a high ratio in the vicinity of the surface of the molded product as illustrated in FIG. 8 of Japanese Patent Publication Laid-Open No. 2-182803 (1990) and (iii) the fact that the concentration of the polymer in the aqueous solution within the void passages of the molded product is thought to be at least equal to the concentration of the polymer at the surface of the molded product although this is not clearly stated in the publications, the water-soluble polymer moves and diffuses slowly during the elementary processes (I) and (II), resulting in a failure in archiving a satisfactory water solvent extraction rate. Known techniques for solving this problem are (i) a concentration controlling means is provided to keep the concentration of the polymer in the aqueous solution low; (ii) a sufficient amount of water is kept; and (iii) fluidization, agitation and vibration are imparted to the aqueous solution. The principle and system of extraction and lixiviation for wet refining are concretely described in "A New Metallurgical Course" New Version Refining Part (Chapter V: Non-Ferrous Metal Refining). This literature quantitatively explains the effect of agitation in the description relating to rates. Also, it is well known that highly viscous aqueous solutions containing a binding agent dissolved therein have such an inherent characteristic that their viscosity is decreased by agitation and fluidization. However, it should be noted that these promoting techniques are merely supplementary, because they leave much to solve. For instance, uniform extraction cannot be achieved even by agitation in molded products of some configurations particularly in their bores, groove bottoms and narrow drill holes. In cases where a plurality of molded products undergo multiple processes, there arises a problem in the arrangement of molded products. Further, rigorous agitation cannot be applied to light-weight molded products and to parts whose shapes are too unstable when they are placed in a furnace for degreasing. Therefore, there have been strong demands for the development of a binder composed of water-soluble substances that are capable of restricting the viscosity of an aqueous solution and it is desirable to use such a binder in combination with the above-described supplementary techniques in the degreasing process.
Of the above supplementary techniques, the fluidization of a water solvent can be attained by (i) splaying a water solvent on a molded product with a nozzle (Japanese Patent Publication Laid-Open No. 2-182803 (1990)); (ii) use of a conveying pump (Japanese Patent Publication Laid-Open No. 2-182804 (1990)); or (iii) use of ultrasonic waves (Japanese Patent Publication Laid-Open No. 2-182804 (1990)). These means are however rather infeasible in cases where a number of molded products are processed at the same time. In fact, it is very difficult in practical operation to set a number of molded products in relation to a nozzle and to bring a number of molded products in uniform contact with water fluidized by means of a conveying pump or ultrasonic waves. Further, these means cannot cope with the case where a plurality of kinds of molded products different in extraction time are continuously processed within the same water solvent extraction vessel.
Another method is disclosed in U.S. Pat. No. 4,197,118 in which a component, e.g., liquid oil is extracted with an organic solvent such as methylene chloride. This method, however, raises serious questions of safety and environmental problems. Further, the strength property of molded products is considerably degraded by use of a liquid oil so that molded products are susceptible to damage when taken out of molds, which limits the shape of molded products to be produced like other methods described earlier.
There are several problems in degreasing of an injection-molded product formed from metal powder. One of the problems is the amount of carbon remaining in the resultant degreased product which relates to the thermal decomposition of a binder. Another problem is cracking caused by the oxidation of the metal powder when degreasing is performed under an oxidation atmosphere. A further significant problem is variations in the sintering density of a sintered product. This is caused by a failure in adequately adjusting the components or carbon content of the sintered product. Some measures are reported to solve these problems, which are: (i) A choice of a suitable binder for injection molding; (ii) degreasing and composition adjustment carried out in an atmosphere of an N.sub.2, Ar or H.sub.2 gas; and (iii) adjustment of atmospheric gas during sintering. These methods are disclosed in Japanese Patent Publication Laid-Open Nos. 5-331503 (1993), 6-200303 (1994) and 6-73406 (1994).
As obvious from Japanese Patent Publication Laid-Open No. 7-305101, it is known that when degreasing is carried out under an atmosphere of an N.sub.2 gas that is inactive for metal powder, the carbon content of sintered iron products considerably differs according to the maximum temperature in completion of degreasing as well as the composition of the binder used. In any cases, it is difficult to adjust the residual carbon content in an atmosphere of an inert N.sub.2 gas or Ar gas. It is reported that when removal of a binder is carried out in the atmosphere, thermal decomposition of a thermoplastic binder is more accelerated with increasing temperature from 200.degree. C., compared to the case of thermal decomposition degreasing under an N.sub.2 gas atmosphere, but degreasing speed drops in the region after temperature reaches 250.degree. C. because of formation of residual carbon, and that the formation of residual carbon creates internal stress, making the molded product more liable to cracking. It is also reported that if a metal powder susceptible to oxidation is used, an oxide is newly created on the surface of the metal, with the result that the molded product is expanded during degreasing and therefore more likely to crack.
Further, when degreasing is carried out in the atmosphere, the residual carbon content becomes substantially zero or oxygen remains as an oxide, or the amount of carbon originally contained in the metal powder decreases. Therefore, a troublesome adjustment is involved, that is, the carbon content of the sintered product has to be adjusted by carefully selecting the type of a powder to be used, the type of a binder to be used, compounding ratio, degreasing conditions and others.
Miura et.al. has proposed a method for adjusting the residual carbon content by carrying out degreasing in an atmosphere of an N.sub.2 gas mixed with a H.sub.2 gas at a high ratio.
It is also contemplated from the difference in the residual carbon content between the case of degreasing in an atmosphere of an N.sub.2 gas and the case of degreasing in the atmosphere that the carbon content can be adjusted by mixing a flow of inert gas such as N.sub.2 with air or oxygen. The details of one example of the above method are reported by "JOURNAL OF THE JAPAN SOCIETY OF Powder and Powder Metallurgy" (Vol.40, No.4,388). This method controls the carbon content by adding hydrogen to N.sub.2 gas. According to this method, when degreasing is carried out at a temperature of 400.degree. C. (this temperature is a very common degreasing condition), a large amount of hydrogen is required to be added to N.sub.2 gas for controlling the carbon content, so that handling of hydrogen enriched gas to be taken out of the furnace is very dangerous. In consequence, it becomes necessary to control the evaporated and removed binder and employ a security system for preventing explosion. Another disadvantage is that the effect of H.sub.2 gas for controlling the carbon content is small.
The method disclosed in Japanese Patent Publication Laid-Open No. 6-200303 suffers from similar problems.
In the method in which air or oxygen is directly added as disclosed in Japanese Patent Publication Laid-Open No. 5-331503, the carbon content of the degreased product is controlled by the direct reaction (this reaction is described by the following formula) between oxygen and carbon which starts to remain in the degreased product. The principle of this method differs from that of the above-described method in which hydrogen is utilized, and it is anticipated that with this method, the residual carbon content can be more effectively controlled compared to the hydrogen addition method. EQU C(S)+(1/2)O.sub.2 (G)=CO(G)(S): solid, (G): gas! EQU G=-26,700-20.75TT: absolute temperature K!
In reality, the direct oxidation reaction between oxygen and carbon is a violent exothermic reaction and particularly when oxygen is added into an inert gas such as N.sub.2 in a slight amount, the reaction of oxygen proceeds more violently according to Le Chatelier's low. Therefore, when degreasing a number of molded products, this method has difficulty in controlling the carbon contents of a number of molded products so as to be uniform and cannot avoid the possibility of occurrence of cracks in molded products which are directly exposed to the introduced gas because of the excessive oxidation reaction. This is the case with the degreasing technique disclosed in Japanese Patent Publication Laid-Open No. 6-192706 (1994) wherein degreasing is performed under an atmosphere of oxygen-enriched air.
The present invention has been made taking the foregoing drawbacks into account, and one of the objects of the invention is therefore to provide a water solvent extraction degreasing method for removing a powder-injection-molding binder that is capable of reducing degreasing time without consideration of abrupt decomposition/vaporization and expansion caused by heat.
Another object of the invention is to provide a degreasing method comprising a water solvent extraction step for extracting a powder-injection-molding binder that is capable of reducing degreasing time while ensuring the shape retention of molded products during the degreasing step.
Another object of the invention is to provide a degreasing method comprising a step for economically, quickly and uniformly performing water solvent extraction on parts of various shapes with a simple system, such parts being formed from a stable, powder-injection-molding binder free from swelling and hydrolysis caused by water and aqueous solution components which are capable of restricting the viscosity of an aqueous solution extracted using water as a solvent.
Still another object of the invention is to provide a degreasing method for degreasing a powder-injection-molding binder that is excellent in strength and toughness in order to reduce injection molding and degreasing defects.
A further object of the invention is to provide a degreasing method in which the amount of residual carbon is adjusted during degreasing, whereby occurrence of defects in degreasing can be prevented and the amount of residual carbon in the resultant sintered product can be controlled.